Thermionic valve



A. H. COOPER THERMIONIC VALVE Filed June 21, 1934 wvmmc By W Patented Oct. 26, 1937 UNITED. STATES PATENT OFFICE 2,097,270 THERMIONIC VALVE Application June 21, 1934, Serial No. 731,598 In Great Britain June 21, 1933 Claims.

The present invention relates to thermionic valves.

The invention is more particularly concerned with superheterodyne wireless receivers, and 5 among its objects is to provide a valve, and a circuit arrangement embodying that valve, for use in the frequency changing, or first detector, stage of such a receiver. It must beunderstood, however, that the invention is not limited to its application to superheterodyne wireless receivers;

other applications will be suggested by the following specification to those skilled in the art.

It is known to provide in the frequency changing stage of a superheterodyne receiver a single evacuated envelope containing the elements of two valves, a triode and a tetrode, the triode being adapted to act as local oscillator and the tetrode operating as first detector and having its screen grid connected to the anode of the triode through an impedance.

According to the present invention a thermionic valve comprises in the order mentioned an electron emitting cathode, a first grid adapted to operate as control grid, a second grid and an anode wherein there is associated with said cathode and second grid a third grid adapted to constitute therewith the elements of a triode valve. V

In a preferred construction the cathode and the second grid are of greater length than the first grid and the third grid is arranged to cooperate With portions of the cathode and second grid which project beyond the first grid. The portion of the second grid which forms the anode of the triode may however be formed as an electrode separate from the main portion of the second grid and may be connected to the main part of the grid by a path of relatively negligible impedance.

According to a feature of the invention, a thermionic valve circuit, for use for example in a frequency changer in a superheterodyne wireless receiver, incorporates a valve according to the present invention as set forth above, wherein the triode portion of the valve is arranged for the generation of electrical oscillations.

The invention will be described by way of example with reference to the accompanying drawing wherein Fig. 1 represents diagrammatically an arrangement of electrodes of a thermionic valve according to the present invention and V Fig. 2 is a circuit diagram of the frequency changing stage of a superheterodyne wireless receiver embodying features of the present invention.

Like parts in the two figures are indicated by like references.

Referring to Fig.1 of the drawing, a rod shaped cathode l of the indirectly heated type, the heating filament of which is provided with leads 2, is surrounded over the greater part of its length by a helically wound first grid 3 adapted to operate as the main control grid. A relatively short portion of the cathode projects beyond the grid 3. The pitch of the helical winding of the grid 3 varies along its length in a manner well known per se: by this means, variation of the mutual conductance of the portion of the valve of which the grid 3 forms one electrode may be obtained by variation of the bias applied to this grid relatively to the cathode.

A second grid 4, constituting a screen is arranged around the main control grid 3 and cathode I and is made to extend beyond that end of the control grid from which the cathode protrudes. An anode 5, which may form a part of the envelope of the valve, is made of substantially the same length as the grid 3 and is located outside the screening grid 4. Surrounding the portion of the cathode l which projects beyond the grid 3 is a third grid 6 adapted to constitute with the projecting portions of the cathode and grid 4 the elements of a triode valve, the grid 6 being constructed as the control grid of the triode and the projecting portion of the grid 4 forming the anode thereof.

The electrodes mentioned above are so disposed that the main part of the valve has characteristics similar to those of the so-called variable-mu screened grid valve.

A fourth grid 1, the purpose of which will be described later, is located between the grids 3 and 4.

A fifth grid 8 which may be connected to the cathode, is located between the grid 4 and the anode. By this or other means, such for example as are described in my co-pending applications Ser. Nos. 737,192 and 24,482, the flow of secondary emission current from the anode 5 to the screen 4 is substantially prevented.

The method of mounting the various electrodes mentioned does not form part of this invention, and is not therefore described in detail. It will be understood that any suitable assembly may be adopted; for example, the electrodes may be mounted on support wires which are held in a pinch, the electrode assembly being mounted in an evacuated glass envelope. Alternatively, as already suggested, the anode of the valve may constitute a part of the envelope of the valve. The valve is preferably mounted on a base of insulating material, the various electrodes being brought out to contact pins.

It will be seen that the valve described is in effect adual valve, and it will be apparent that the valve may be employed for many purposes for which two valves have previously been used. As an example of such an application, the use of the valve in the frequency changing, or first detector, stage of a wireless receiver will be described.

Referring to Fig. 2 of the drawing the heater terminals 2 of the indirectly heated cathode I are connected to a source of alternating current and the cathode is earthed through a suitable biasing resistance 9, shunted by a by-pass condenser It). The function of this resistance is discussed below. The first grid 3 which operates as the main control grid is connected through a tunable circuit comprising an inductance H and a variable condenser [2 in parallel to a terminal l3 towhich is connected the negative pole of a source of variable grid bias (not shown) thepositive pole of which is earthed. The circuit ll, I2 is tunable over the range of signal frequencies to be received, and iscoupled, either directly or through a high frequency amplifier, to the aerial circuit of the receiver by means of the inductance l4. The grid bias applied to the main control grid 3 may be arranged, in any known or suitable manner, to vary in accordance with variations in the strength of the received signal, in order to provide automatic gain control.

The auxiliary control grid 6 of the valve is connected through a grid leak I5 to earth, and through a grid condenser 16 to one terminal of a parallel tunable circuit, the other terminal of which is earthed. The tunable circuit comprises an inductance coil I! in parallel with a variable condenser l8 and the inductance coil I1 is mutually coupled to a second coil l9 arranged between the screening grid 4 and the positive terminal of a source of high tension current (not shown) the negative terminal of the source being earthed.

The arrangement is such that oscillations are generated in the circuit comprising the cathode l, the auxiliary control grid 6 and the screening grid 4 (acting as anode), and the potential of the screening grid 4 relatively to the cathode therefore fluctuates at the frequency of the generated oscillations. Thus the potential of the main control grid 3 fluctuates at the signal frequency, while the screening grid 4 fluctuates at the frequency of the locally generated oscillations, and the corresponding fluctuations of potential of the anode 5 are found to contain a component at the intermediate frequency. It will be noted in the arrangement described that the beating or mixing of the two oscillations has been attained without use being made of any non-linearity of the characteristic of the pentode part of the valve. This part of the valve may therefore be operated on a substantially straight part of the grid potential anode current characteristic although this characteristic is preferably in the form of a smooth curve in order that smooth gain control may be obtained by variation of the bias on the grid 3.

The anode 5 is connected to the positive terminal of a source of high tension current through a parallel tuned circuit 20, 2|. This circuit is tuned to the intermediate frequency and is coupled to the input circuit of an intermediate frequency amplifier which is not shown, by means of the inductance 22.

In the absence of the biassing resistance 9 mentioned above, as the bias applied to the main control grid 3 is increased, the impedance of the path between the grid 4 and the cathode in the pentode part of the valve increases, and the conditions governing the generation of oscillations by the triode part change accordingly. On account of this fact, difficulty may be experienced in making the oscillator stable over a wide frequency range.

To obviate this disadvantage, the auxiliary control grid 6 is given a small negative bias with respect to the cathode l by the introduction of the biassing resistance 9 referred to. As the anode current decreases on account of an increase in the main control grid bias, the second grid 6 becomes more positive with respect to the cathode, and the damping thereby introduced compensates for the increase in the impedance of the path between the grid 4 and the cathode I.

Since the potential of the screening grid 4 fluctuates at the frequency of the local oscillations, there is a possibility of electrostatic feed back of oscillations of this frequency into the signal input circuit. If the input circuit be directly coupled to the aerial, these oscillations will be radiated and may cause interference with neighboring receivers.

In order to eliminate this possibility the fourth grid I is maintained at a suitable constant positive potential, the grid 3 being thereby screened from the grid 4.

It will be seen from the above description that the valve according to this invention may be used in a variety of circuits such for example as modulation circuits, homodyne circuits and the like.

I claim:

1. A thermionic valve comprising, in the order named, an electron emitting cathode, a first control grid, a second grid and a main anode, said first control grid and said second grid disposed in the path of a main electron stream from said cathode to said main anode, an auxiliary anode directly connected to said second grid and located substantially clear of said path, a second control grid arranged to control an auxiliary electron stream from said cathode to said auxiliary anode and a screen grid interposed between said first control grid and second grid and having a length substantially less than said second grid whereby said screen grid has substantially no influence on the control of said auxiliary electron stream by said second control grid.

2. A thermionic valve comprising an elongated cathode, a first control grid, a screening grid and an anode, said cathode, said grids and said anode arranged in the order named, said screening grid and cathode having portions extending beyond said anode and said first control grid, and a second control grid disposed between said portions.

3. A thermionic valve comprising an elongated cathode surrounded by a first control grid, a screening grid and an anode, said cathode, said grids and said anode arranged in the order named, said screening grid and cathode having portions extending, in a direction parallel to the length of said cathode, beyond said anode and said first control grid, and a second control grid located outside the electron stream between said cathode and anode, and a third control grid interposed between said anode and first control grid and also having a portion arranged to surround said second control grid.

5. The arrangement of claim 4 in which a suppressor grid having a length substantially less than said third control grid is interposed between said anode and third control grid.

ARTHUR HENRY COOPER. 

